1. Field of the Invention
This invention relates to a manufacturing process for plate or forging (bar, stamp work or the like) of ferrite-austenite two-phase stainless steel and particularly of ferrite-austenite two-phase stainless steel superior in resistance to nitric acid.
2. Description of the Prior Art
In a nitric acid environment, a stainless steel having a higher content of Cr is strong in resistance thereto accordingly, and an intergranular corrosiveness is extremely severe according to the density of nitric acid, therefore an extremely-low carbon type and Nb-stabilized high-chrome austenite stainless steel, 310 LC (low carbon--25% Cr--20% Ni steel), 310 LCNb (low carbon--25% Cr--20% Ni--0.2% Nb steel) or the like, for example, is employed hitherto. However, in the case of such austenite stainless steel having a higher content of Ni, since a solid solubility limit of carbon (C) is small, a chrome carbide deposits preferentially onto a crystal grain boundary to deteriorate intergranular corrosion resistance under the effect of heating at 500.degree. to 900.degree. C. or welding heat, and a solidification cracking sensitivity is high at the time of welding, thus losing a reliability on the weld zone. On the other hand ferrite-austenite two-phase stainless steel having a high Cr content is susceptible, due to the heat generated by welding, to selective corrosion between the structures. Such corrosion tendency is conspicuous particularly in a nitric acid environment, and thus a conventional two-phase stainless steel has two properties adversely impacting on its ability to work as a nitric acid resistant material having a welded structure.
As the result of having studied on influences of structure and percentages of elemental fractions on nitric acid resistance in stainless steel, the inventors contrived a high-chrome two-phase stainless steel effective to alleviate the above-described defects of austenite stainless steel and two-phase stainless steel, and provide superior in nitric acid resistance and weldability, and cheap in cost as well. (Japanese Patent Application No. 130442/1981 (Japanese Patent Laid-Open No. 3106/1983)). This inventive type of steel has a higher Cr and Ni content as compared with a conventional ferrite-austenite two-phase stainless steel having Cr of 23 to 25% and Ni of 4 to 6% generally, and a specific Ni balance value at the same time. Moreover, a structure has been found which is superior in nitric acid resistance to the above-mentioned materials of 310 LC and 310 LCNb even though an expensive Ni component is present in lesser amounts. Further, the nitric acid resistance is improved by adding B at 0.001 to 0.03% thereto, which is enhanced more by decreasing P to 0.010% or below and S to 0.005% or below which are contained inevitably as impurities, and it has the following compositions:
(1) That for which the composition is: C being 0.03% by weight or below, Si being 2.0% or below, Mn being 2.0% or below, P being 0.040% or below, S being 0.030% or below, Cr being 25 to 35%, Ni being 6 to 15%, N being 0.35% or below, Fe and inevitable impurity for the remainder, and also the following expression EQU -13&lt;Ni eq-1.1.times.Cr eq+8.2&lt;-9 PA1 (2) That for which 0.001 to 0.03% B is added to the above mentioned steel. PA1 (3) That for which P and S are decreased independently or simultaneously to 0.010% or below and to 0.005% or below respectively in the above mentioned steels (1) and (2).
is satisfied.
The inventive steel has superior resistance to nitric acid, this property is the result of the elemental composition and the fine grain structure of ferrite and austenite peculiar to two-phase stainless steel. That is, the superior resistance to nitric acid is due to a superior intergranular corrosion resistance, and it is generally known that the intergranular corrosion resistance depends on a crystal grain size, and the smaller the crystal grain size is, the better the resistance becomes. Thus the superior intergranular corrosion resistance of the steel is intrinsically related to the fine structure which is a feature of the two-phase stainless steel. Originally, the crystal grain size of the two-phase stainless steel is influenced largely by its manufacturing history, and the larger a forging ratio is, the smaller the grain size becomes. However, when it is heated at high temperatures of 1,250.degree. C. and over for hot working, the structure comes near to a single phase structure of ferrite with the resulting excessive coarsening of the crystal grain.